Comment

Only completely blind bats, which isn't many species. At dawn and dusk, eyes come in handy, even for bats, because they give much higher resolution vision than sonar "eyes". If there is even a low level of light, therefore, animals (ie, organisms that aren't rooted in the ground) will evolve eyes to see it, even if they also have any number of other senses.

Agreed, but we are hypothesising here on the potential need for the evolution of eyes on an alien world. I'm reminded of the movie "Pitch Black" for some reason. It is not beyond the realm of possibility that a sonar type "sight" could advance over light sensitive sight....what we "see" is merely the mapping of light waves on a model in our brains caused by electrical signals via the eyes, RD suggests that Bats probably do the same with sound waves to the point they can hear "colour" and "texture". He actually uses a hypothetical race of sonar "seeing" aliens discussing and debating the complexity and understanding of the invention of an artificial light operating seeing device, a good way of expressing the idea as well as amusing as is the case in his books.

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